Method for forming a high-gradient magnetic field and a substance separation device based thereon

ABSTRACT

The invention relates to a magnetic separation device and is used for separating paramagnetic substances from diamagnetic substances, the paramagnetic substances according to the paramagnetic susceptibility thereof and the diamagnetic substances according to the diamagnetic susceptibility thereof. Said invention can be used for electronics, metallurgy and chemistry, for separating biological objects and for removing heavy metals and organic impurities from water, etc. The inventive device is based on a magnetic system of an open domain structure type and is embodied in the form of two substantially rectangular constant magnets ( 1, 2 ) which are mated by the side faces thereof, whose magnetic field polarities are oppositely directed and the magnetic anisotropy is greater than the magnetic induction of the materials thereof. Said magnets ( 1, 2 ) are mounted on a common base ( 4 ) comprising a plate which is made of a non-retentive material and mates with the lower faces of the magnets, thin plates ( 5, 6 ) which are made of a non-retentive material, are placed on the top faces of the magnets and forms a gap arranged above the top edges ( 8, 9 ) of the magnets ( 1, 2 ) mated faces. A nonmagnetic substrate ( 10 ) for separated material ( 11 ) is located above the gap ( 7 ).

TECHNICAL FIELD

The invention relates to methods and devices of magnetic separation and it is intended for: a) the separation of paramagnetic substances from diamagnetic ones, b) the division of paramagnetic substances depending on their paramagnetic susceptibility, and c) the division of diamagnetic substances depending on their diamagnetic susceptibility. Possible fields of application of the invention are production of clean and super pure substances and materials in electronics, metallurgy and chemistry, separation of biological subjects (red blood cells, “magnetic bacteria”, etc.) in biology and medicine, removal of heavy metals and organic impurities from water, etc.

BACKGROUND ART

The basic factor of magnetic separation is the magnetic force, which acts on a particle of the substance and which is proportional to the magnetic susceptibility of the substance, the value of the magnetic induction B and the value of the gradient ∇B of the applied magnetic field. Therefore, increasing the sensitivity and selectivity of magnetic separation will require use of the highest possible values of magnetic induction and magnetic field gradient, or their united factor—the product B∇B.

It is known a magnetic separator intended for the separation of ferromagnetic materials in terms of the values of their magnetic susceptibility which makes it possible to reach a value of the product B∇B of about 4.5·10⁵ mT²/m in a gap of a few millimeters [1]. However, this magnetic separator cannot be used for the separation of paramagnetic and diamagnetic substances and materials, because the values of the magnetic field parameters are not high enough.

It is known a magnetic system which consists of two permanent magnets with opposite magnetization in the form of a Kittel open domain structure [2]. In this system, near the edges of the faces of the joining magnets, a strong magnetic stray field appears which is caused by the non-diagonal matrix elements of the demagnetization factor tensor (see FIG. 1), and the value of the product B∇B reaches 10¹¹ mT²/m. On the surface of magnets, in the zone of the upper edges of the joining faces (in the zone of line 0Y in FIG. 1), a strong magnetic stray field appears with the components Hy(x,z), Hz(x,z) and Hx(x,z). The component Hy(x,z) is equal to zero due to the geometry of the system, the vertical component Hz(x,z) comprises less than half the value of the induction of the magnet material, and the horizontal component Hx(x,z), which in the present case is of greatest interest, can be described by the expression: Hx(x,z)=Ms [ ln(a ² +z ²+2ax+x ²)−2 ln(x ² +z ²)+ln(a ² +z ²−2ax+x ²)], wherein:

-   -   Ms is the magnetization saturation of the magnets, and     -   a is the size of the magnet along the 0x axis (see FIG. 1).

It follows from this expression that on the plane z=0, at point 0 the horizontal component of the stray field strives into infinity. As a result, in a small area −0.1a≦x≦0.1a, along the line of the joining magnets the horizontal component of the magnetic stray field makes an abrupt jump, which is noted by a dotted line in FIG. 1, the intensity of which can be several times stronger than the induction of the magnet material.

The important practical feature of the magnetic system described is the fact that the stray field Hx(x,z) possesses a high gradient, which in the area near to the point 0 can reach a values of 10⁶-10⁹ mT/m. In this system the value of the product B∇B reaches 10¹¹ mT²/m. The disadvantage of this magnetic system is the impossibility of controlling the form and gradient of the created magnetic fields which causes the practical impossibility of using this system for the separation of substances and materials.

A high-gradient magnetic separator is known, which makes it possible to reach a value of the product B∇B of about 1.3·10¹⁰ mT²/m in a gap of a few micrometers [3]. The disadvantage of this separator is the necessity of introducing ferromagnetic bodies, (wires, balls, and the like) with a size of 25-60 μm into the substances being analyzed, this fact substantially limiting the possible range of properties and characteristics of the substances to be separated.

A device for continuous removal of impurities from colloidal dispersions, which contain pathogenic components, such as viruses and microbes, is known [4]. The device is supplied with at least one magnet with a central core, the poles of which are turned to one another and located in such a way that they form a channel with a magnetic field, which is perpendicular to their surfaces. In the channel there is a basket in the shape of a tray of rectangular cross-section and made from non-magnetic material, in which a filter is established from a material with high magnetic permeability, in the form of untied fibres, wires, net-like cloths or powders, which makes it possible to create a high gradient magnetic field. One side of the basket and filter communicates with a chamber for supplying the solution, and the other—with a chamber for collecting the filtered liquid. The disadvantage of this device is the necessity of introducing ferromagnetic bodies in the form of the filter, into the substances being analyzed and the impossibility of its application for the separation of non-liquid substances.

A magnetic system is known, for magnetic separation of biological substances by the method of sedimentation of particles, which can be magnetized, from the suspension [5]. This magnetic system includes a carrier plate, on which an iron plate is fixed, and a number of permanent magnets mounted on the iron plate, the polarity of each magnet being opposite of the polarity of the adjacent magnet. A magnetic field concentrator plate of iron is overlying the magnets and a cover plate is disposed above the field concentrator plate. A hole is provided in the cover plate and field concentrator plate for locating in the magnetic field, tubes with the suspension being separated. The plate of the magnetic field concentrator has a smooth external surface and a cone-shaped cross-section, such that the thickness of the plate decreases towards the holes. The disadvantage of this magnetic system is the impossibility of achieving such parameters of the magnetic field that would allow using it for the separation paramagnetic substances in terms of the magnitudes of their paramagnetic susceptibility.

DISCLOSURE OF INVENTION

The device according to the present invention is designed in order to solve the problem of creating strong and high gradient magnetic fields with adjustable form and a gradient in the zone of separation, for use as a high-sensitivity magnetic separator for separation of different types of paramagnetic substances and materials from diamagnetic ones, for division of the paramagnetic substances and materials in terms of the magnitudes of their paramagnetic susceptibility, and also for division of the diamagnetic substances and materials in terms of the magnitudes of their diamagnetic susceptibility.

This aim can be reached by the presented method of creating a high gradient magnetic field, which is formed in the Kittel open domain structure above the free edges of the mating faces of two magnets with opposite directions of the polarity of the magnetic field, the magnetic anisotropy of which substantially exceeding the magnetic induction of the magnet material. The dimensions of the zone are set by thin magnetic soft-iron plates, which are placed on the free faces of the magnets such that they form a narrow gap located immediately above the upper edges of the mating faces of the magnets.

This problem is solved also by the fact that the device for magnetic separation of substances is based on a magnetic system made as an open domain structure which consists of two permanent magnets, the lateral sides of which are joined, the shape of the magnets, as a rule, being rectangular with opposite directions of their magnetic field polarity, and their magnetic anisotropy substantially exceeding the magnetic induction of the magnet material. The magnets are mounted on a common base which includes the magnetic plate made from soft-iron material and joined with the lower sides of the magnets. On the upper sides of the magnets thin plates of magnetic soft material which form a narrow gap, are located immediately above the upper edges of the mating faces of the magnets, and immediately above the gap, a non-magnetic substrate for the material being separated.

In a particular embodiment of the invention the thin plates are made of a magnetic soft material, such as vanadium permendur.

In another particular embodiment of the invention the thin plates are made with a thickness from 0.01 to 1.0 mm.

In another particular embodiment of the invention the thin plates are provided with means for their displacement along the surfaces of the upper sides of the magnets in order to regulate the size of the gap between 0.01 and 1.0 mm, located symmetrically relative to the plain of the joining magnets.

In another particular embodiment of the invention the substrate is made as a thin band or tape of non-magnetic material, such as polyester.

In another particular embodiment of the invention the band is provided with means for its displacement along a direction perpendicular to the longitudinal axis of the gap.

In another particular embodiment of the invention the substrate is made as a non-magnetic plate connected to a source of mechanical oscillations.

In another particular embodiment of the invention the magnets are made of such materials as Nd—Fe—B, Sm—Co, or Fe—Pt.

In another particular embodiment of the invention the device is formed on the basis of two or more magnetic systems as a series of joining faces of three or more magnets, the zones of separation having the form of two or more slots above the upper edges of the mating faces.

The upper edges of the mating faces of the magnets are the zones of magnets which directly adjoin the line of intersection of two planes, one of them being the plane along which the lateral sides of magnets are mated, and the other the plane of the upper sides of the magnets (see numerals 8 and 9 in FIG. 6).

The main feature of the device according to the present invention is the ability to considerably increase the magnitude of the product B∇B in the zone of separation and also regulate the product B∇B, which gives the practical possibility of using the high magnetic stray fields for the creation of a high-sensitivity magnetic separator.

The illustrations in FIGS. 2 and 3, and also FIGS. 4 and 5, demonstrate the change in the magnetic field configuration compared to the known open domain structure [1], that is achieved due to the invention. The presented illustrations show that with the magnetic system according to the invention it is achieved not only a concentration of the magnetic field in the zone formed by the gap between the plates, but also a change in the shape of the magnetic force lines, as well as in the magnitude and distribution of the magnetic induction nearby the edges of the joined sides of the magnets. Thus, the invention makes it possible to change the parameters of the magnetic field considerably, and to create the most suitable conditions for the separation of materials over a wide range of their magnetic properties, including the separation of paramagnetic substances and materials in terms of the magnitudes of their paramagnetic susceptibility, and the separation of diamagnetic substances and materials in terms of the magnitudes of their diamagnetic susceptibility.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an illustration of the Kittel open domain structure of two magnets,

FIG. 2 presents a schematic diagram of the magnetic force lines in the Kittel open domain structure,

FIG. 3 presents a schematic diagram of the magnetic force lines in the magnetic system according to the present invention,

FIG. 4 is a graph showing the variation in the horizontal component of the magnetic induction nearby the edges of the joined magnets in the Kittel open domain structure,

FIG. 5 is a graph showing the variation in the horizontal component of the magnetic induction nearby the edges of the joined magnets in the magnetic system according to the present invention,

FIG. 6A is an illustration of the of the magnetic system according to the present invention, and FIG. 6B is an illustration of part of the magnetic system in accordance with one or more embodiments of the invention,

FIG. 7 is a graph showing the dependence of the magnetic field induction in the gap zone, on the distance from the surface of the plates, and

FIG. 8 is an illustration of the magnetic system in accordance with one or more embodiments of the invention.

DESCRIPTION OF PREFERRED EMBODIMENT

The disclosed device (see FIG. 6) consists of two magnets 1 and 2 of a predominantly rectangular shape, with opposite directions of magnetization (shown by arrows in the figure). The magnets are made of a material with a much greater magnetic anisotropy than the induction of a material of magnets, such as neodymium-iron-boron, ironplatinum or samarium-cobalt, for example.

In experiments sintered neodymium-iron-boron magnets were used with a remnant induction of about 1.3 T, an intrinsic coercive force of magnetization of about 1300 kA/m, and a maximum energy product of about 320 kJ/m³. The size of magnets was 25×50×50 mm.

The magnets 1 and 2 are joined together along a plane 3 and and their lower sides placed on a basis 4 in the form of a plate made of soft-iron material, for example, with a thickness of 5-25 mm.

On the upper sides of the magnets 1 and 2, thin plates 5 and 6 are located which are made of a magnetic soft material with high magnetic saturation induction, their thickness being 0.01-1.0 mm. The thickness of plates 5 and 6 should be chosen depending on the required magnitudes of the magnetic induction and the optimum field gradient for the separation of real substances and materials. The plates 5 and 6 are located on the upper sides of the magnets 1 and 2 with a clearance forming a narrow gap 7 which is 0.01-1.0 mm wide immediately above the upper edges 8 and 9 of the magnets 1 and 2, as a rule, symmetrically relative to a plane 3. Immediately above the gap 7 there is a non-magnetic substrate 10 for the placing of the material being separated 11. The substrate 10 can be made as a horizontal plate, for example, connected to a generator of mechanical oscillations. The substrate can also be made as a thin non-magnetic, band 14 (of polyester, for example) and be provided with means to move the band 15 along a direction perpendicular to the longitudinal axis of the gap 7 (the band and its moving means are shown in FIG. 6B). The substrate 10 can be provided with means to displace it a distance of 0-5 mm from the surface of the plates 5 and 6. The plates 5 and 6 are connected to the means 12 and 13 for moving them along the upper sides of the magnets 1 and 2 in order to regulate the width of the gap over a range of 0.01-1.0 mm.

The device makes it possible to create strong magnetic fields with a magnitude of the product B∇B of more than 4·10¹¹ mT²/m at a distance less than 10 μm from the surface of the plates 5 and 6, forming the gap. Thus, for a particular embodiment of the device, where vanadium permendur plates with a thickness of 0.20 mm are being used and the gap width is 0.05 mm, the tangential component of the magnetic field induction exceeds 4.0 T. Furthermore, the peak width of the magnetic field tangential component can be regulated by the width of the gap 7.

FIG. 7 shows the dependence of the magnetic field induction on the distance from the axis perpendicular to the plane of the plates 5 and 6. The origin of coordinates in FIG. 7 corresponds to a point in the center of the gap 7 at the level of the plates 5 and 6. At a distance of 0.10 mm from this point the gradient is 4.1·10⁶ mT/m, and at a distance of 0.01 mm 1.2·10⁸ mT/m, while the product B∇B is 4.2·10¹¹ mT²/m.

The experimental examination of the possibility to separate paramagnetic substances using the disclosed device was carried out on a mixture of substances with different paramagnetic susceptibility. The results are presented in the following table.

TABLE 1 The separation of a mixture of substances with different paramagnetic susceptibility Substance Susceptibility [χ · 10⁶] Distance [mm] Dysprosium sulfate 92760 1.900 Europium chloride 26500 0.700 Copper chloride 1080 0.100

The separation process was conducted as follows: The mixture of the substances presented in the table above, was placed on a thin polyester band, which was located at a fixed distance from the plates 5 and 6. Then the band was moved above the surface of the plates along a direction perpendicular to the longitudinal axis of the gap 7. The particles of dysprosium sulfate, which possess the greater magnetic susceptibility, were separated from the mixture, when the distance between the band and the plates 5 and 6 was about 1.90 mm, while the other particles of the mixture continued to move on together with the band. Then the separated particles of dysprosium sulfate were removed from the band, the distance between the band and the plates 5 and 6 was decreased, and the separation process was continued.

The table presents the magnitudes of distances from the band to the surface of the plates 5 and 6, which correspond to the separation of all the components of the paramagnetic substances mixture.

INDUSTRIAL APPLICABILITY

On the basis of the magnetic system with two magnets according to the invention, a more productive magnetic separator can be created, as a composition of two or more analogous magnetic systems. Each system should be formed by a serial joining of the faces of the three or more magnets, with separation zones in the vicinity of two or more gaps formed by the plates above the upper edges of the mating faces, as shown in FIG. 8. In a system of four magnets and three separation zones a three-stage separation of substances could be executed during one passage of the band with substances being separated.

Thus, the disclosed device makes it possible to create strong magnetic fields with a very high magnitude of the product B∇B, i.e. of more than 4·10¹¹ mT²/m, at a distance less than 10 μm from the surface of the plates forming the gap. The device makes it possible to regulate the shape and gradient of the magnetic field in the zone of separation. In practice, the invention can be used for the separation of paramagnetic substances and materials from diamagnetic ones, for division of paramagnetic substances and materials in terms of the magnitudes of their paramagnetic susceptibility, and for division of diamagnetic substances and materials in terms of the magnitudes of their diamagnetic susceptibility. The substances can be both in the form of powders and in the form of colloidal solutions and suspensions.

BIBLIOGRAPHIC DATA

-   1. Glebov, V. A.; Glebov, A. V.; Knyazev, Yu. D.; Nefedov, V. S.;     Lileyev, A. S: “Magnetic separation of fast-hardened powders of     neodymiumironboron systems”; Proceedings of VUZ—Institute of Higher     Education; Materials of Electronic Engineering, No. 4, 2003, pp.     59-61. -   2. Samofalov, V. N.; Ravlik, A. G.; Belozorov, D. P.; Avramenko, B.     A.: “Strong magnetic fields of scattering in systems from the highly     anisotropic magnetic materials”, Physics of Metals and Metallurgical     Science, 2004, Volume 97, No. 3, pp. 15-23. -   3. Gh. lacob, Ay. D. Ciochina, O. Bredetean: “High Gradient Magnetic     Separation Ordered Matrices”, European Cells and Materials, Vol. 3.     Suppl. 2, 2002 25 (pp. 167-169), ISSN 1473-2262. -   4. European patent No. 0 429 700, published May 4, 1995. -   5. European patent No. 0 589 636, published Feb. 8, 2000. 

The invention claimed is:
 1. A device for separating material in a high-gradient magnetic field, the device comprising: two permanent magnets, each of the permanent magnets having an upper side, a lower side, and a lateral side with the lateral sides of the permanent magnets joined together, the permanent magnets having directions of magnetic field polarity being opposite to one another, a magnetic anisotropy of the permanent magnets essentially exceeding the magnetic induction of a material of the permanent magnets, wherein the permanent magnets are mounted on a common base comprising a soft magnetic material that is connected to the lower sides of the permanent magnets, wherein, on the upper side of the permanent magnets, magnetic soft plates that are substantially thinner across a length thereof than each of the two permanent magnets are placed to form a narrow gap located immediately above upper edges of the joined lateral sides of the permanent magnets, wherein a thickness of the magnetic soft plates is 0.01-1.0 mm, and wherein, above the narrow gap, is a non-magnetic substrate for the material being separated.
 2. The device of claim 1, wherein the magnetic soft plates are made of vanadium permendur.
 3. The device of claim 1, wherein the narrow gap between the magnetic soil plates has a width in a range of 0.01-1.0 mm, the gap being located symmetrically about a plane, along which the lateral sides of the permanent magnets are joined.
 4. The device of claim 1, wherein the substrate moves the material along a direction perpendicular to the longitudinal axis of the gap.
 5. The device of claim 1, wherein the substrate is a horizontal plate connected to a generator of mechanical oscillations.
 6. The device of claim 1, wherein the permanent magnets are made of neodymium-iron-boron, samarium-cobalt, or iron-platinum.
 7. The device of claim 1, further comprising: one or more additional permanent magnets, identical to the permanent magnets, wherein the lateral sides of three or more additional permanent magnets and permanent magnets are joined in series to form two or more narrow gaps located immediately above upper edges of the joined lateral sides of the additional permanent magnets and permanent magnets.
 8. The device of claim 2, wherein the narrow gap between the magnetic soft plates has a width in a range of 0.01-1.0 mm, the gap being located symmetrically about the plane, along which the lateral sides of the permanent magnets are joined.
 9. The device of claim 1, wherein the permanent magnets have a substantially rectangular shape.
 10. A device for separating substances in a high-gradient magnetic field, the device comprising: two permanent magnets, each of the permanent magnets having an upper side, a lower side, and a lateral side with the lateral sides of the permanent magnets joined together, the permanent magnets having directions of magnetic field polarity being opposite to one another, a magnetic anisotropy of the permanent magnets essentially exceeding the magnetic induction of a material of the permanent magnets, wherein the permanent magnets are mounted on a common basis that includes a magnetic soft plate connected to the lower sides of the permanent magnets, wherein, on the upper side of the permanent magnets, magnetic soft plates that are substantially thinner across a length thereof than each of the two permanent magnets are placed to form a narrow gap located immediately above upper edges of the joined lateral sides of the permanent magnets, wherein a ratio of a thickness of the magnetic soft plates with respect to a thickness of the two permanent magnets is about 1:25, and wherein, above the gap, is a non-magnetic substrate for the material being separated.
 11. A device for separating substances in a high-gradient magnetic field, the device comprising: two permanent magnets, each of the permanent magnets having an upper side, a lower side, and a lateral side with the lateral sides of the permanent magnets joined together, the permanent magnets having directions of magnetic field polarity being opposite to one another, a magnetic anisotropy of the permanent magnets essentially exceeding the magnetic induction of a material of the permanent magnets, wherein the permanent magnets are mounted on a common basis that includes a magnetic soft plate connected to the lower sides of the permanent magnets, wherein, on the upper side of the permanent magnets, magnetic soft plates that are substantially thinner across a length thereof than each of the two permanent magnets are placed to form a narrow gap located immediately above upper edges of the joined lateral sides of the permanent magnets, wherein the ratio of the thickness of the magnetic soft plates with respect to the thickness of the two permanent magnets is about 1:50, and wherein, above the gap, is a non-magnetic substrate for the material being separated. 